The invention relates to a device for the of clarified waste water from rectangular basins.
In sewage engineering, secondary sedimentation basins having a rectangular plan view are being used to an increased extent in sewage treatment plants. In these basins, the phases are separated by the sedimentation of the heavy sludge particles and the purified waste water undergoes a vertical movement in the direction of the surface.
So that said vertical movement is not disturbed by the flow processes in the basin, the outlet system for the purified waste water should be designed so that a two-dimensionally constant inflow to the outlet system is ensured.
Hitherto, suspended outlet troughs have predominantly been used as outlet systems. Said troughs draw off the clarified waste water as collecting troughs. The flow in the trough is governed by the fact that the throughput increases in the flow direction but the flow depth has to decrease. The openings into the outlet trough have to be adapted to this circumstance, and this means that either the number of openings decreases in the flow direction or, alternatively, the passage area of the openings decreases in the flow direction.
For this reason, the dimensioning of said outlet troughs is difficult because the uniformity of the influx depends on the accuracy of the calculation. Since the transfer velocity is controlled by means of height differences between basin and trough in the centimetre range, the dimensioning is subject to uncertainties.
The suspended troughs having an open surface have, in addition, some disadvantageous properties. On the one hand, they have to be cleaned at great expense. This is a consequence of the growth of algae in the region of the open surface. On the other hand, they have the largest flow area where the velocities are lowest. The water depth at the start of the trough is greatest because of the characteristics of the collecting trough. Wind action affects the flow.